Feeding a boiler with solid fuel require:
- a storage silo
- a fuel material handling system
- a fuel control and metering system
- a adiabatic combustion chamber for the oxidation-reduction process
- a flue gas treatment line
We will not dwell here on aspects related to storage, handling and feeding, even if of fundamental importance for the quality of the final plant. We want to highline on combustion aspects and emission treatment.
We can outline the typical steps of solid biomass combustion process as follows:
- Heating and drying
- Pyrolysis and releasing volatile compounds
- Combustion in the gas phase
- Secondary combustion of carbon residual
- Drain the ashes
It is important to emphasize that the main parameter so that byproducts and pollutants are limited, is the quality and stability of the combustion. This parameter, in general, depends on the level of control and stability that the boiler and the plant can maintain on the same reaction, or by the quality of the design of the boiler and by the use of proper fuel to the installation characteristics. The pollutants that are produced may be treated with a special line of flue gas treatment.
The main controls that a solid biomass combustion system must be able to carry out finely, to manage the quality of combustion reaction, are:
- The amount and distribution of oxygen present during the reaction: this parameter affects the stability of the temperatures, the creation of carbon monoxide, the efficiency and the production of specific pollutants.
- The quantity and the fuel supply mode. This control depends on the presence of an automatic feeding system and is crucial for a correct ignition, for the stabilization of the combustion system, for controlling the supply of heat (by choking the loads), and for a correct temperature control.
- The flue gas recirculation, which allows the control of temperatures and results in a reduction of emissions of nitrogen oxides, affecting in a direct manner on the kinetics of the oxidation reaction. This allows to reduce the upstream contaminant concentration.
On a main plant level, also, it is good to consider:
- The presence of accumulators, giving / accepting their reserves of thermal energy, facilitate the management of peaks / falls of heat demand and thus enabling them to “smooth out” sudden violent changes in the combustion. Such variations in fact, in addition to decreasing the average efficiency of the plant, would contribute to higher pollutant productions.
- Last, but not least important, the aspect of maintenance: both for the part of combustion that, for the part of the abatement systems, must be planned maintenance and, in certain cases, preventive one must be guaranteed.